1. Field of the Invention
The present invention relates to a coaxial plasma processing apparatus for ashing a resist film on a semiconductor wafer, for example.
2. Description of the Prior Art
Japanese patent publication No. 53-33471 discloses a coaxial plasma processing apparatus in which inner and outer cylindrical electrodes are coaxially arranged with a cylindrical chamber interposed therebetween.
As shown in FIG. 1 of the accompanying drawings, the disclosed coaxial plasma processing apparatus comprises a vertically elongate cylindrical chamber 100 of quartz or the like which has upper and lower openings closed by respective chamber plates 101, 102. Inner and outer cylindrical electrodes 103, 104 are coaxially positioned inside and outside, respectively, of the cylindrical chamber 100. The coaxial plasma processing apparatus also has a preheater 105 located outwardly of the outer cylindrical electrode 104.
A wafer holder 106 comprising vertical rods of quartz or the like carries a vertical stack of 50 to 100 wafers 107. In operation, the wafer holder 106 with the stack of wafers 107 held thereby is inserted upwardly into the coaxial plasma processing apparatus through the lower chamber plate 102 in the direction indicated by the arrow A.
Then, the cylindrical chamber 100 is evacuated, and oxygen is introduced into the evacuated cylindrical chamber 100. The preheater 105 is energized to heat the coaxial plasma processing apparatus for thereby providing conditions for generating a plasma in the coaxial plasma processing apparatus. A Radio Frequency generator 108 connected to the outer cylindrical electrode 104 is thereafter energized to generate an oxygen plasma between the outer and inner cylindrical electrodes 104, 103.
The generated oxygen plasma ashes a photoresist on the surfaces of the wafers 107 supported on the wafer holder 106.
FIG. 2 of the accompanying drawings shows the temperature dependency of the ashing rate of each of the wafers 107. The horizontal axis of the graph of FIG. 2 represents radial positions, i.e., an end, a center, and an opposite end of the wafer 107, and the vertical axis the ashing rate. Parameters t0, t1, t2 indicate different processing temperatures, and are of the relationship of t0&lt;t1&lt;t2.
Study of FIG. 2 reveals that the ashing rate is lower at the center of the wafer 107 than at the ends of the wafer 107, and the ashing rate is higher as the processing temperature is higher.
The preheater 105 serves to keep the apparatus at a certain temperature for thereby increasing the ashing rate to a certain level.
When an oxygen plasma is generated in the coaxial plasma processing apparatus, the temperature in the cylindrical chamber 100 tends to rise gradually due to the heat of the plasma. Since the cylindrical chamber 100 is vertically elongate for housing the stack of 50 to 100 wafers 107, the upper portion of the cylindrical chamber 100 becomes hotter than the lower portion thereof because of a draft generated in the cylindrical chamber 100. Therefore, as shown in FIG. 2, those wafers 107 which are positioned higher in the cylindrical chamber 100, and are thus exposed to higher temperatures, suffer greater ashing rate differences between the ends and center of the wafers 107.